362 On a Ternary Geological Classijkation. [July, 



pare this section with that on the west of England (Cumberland 

 and Lancashire), we find the conditions of these several members 

 completely reversed in respect of development. Here the central 

 calcareous member is reduced to very small dimensions, being 

 represented in South Lancashire by marls with thin bands of 

 limestone,* and in Cumberland by thin bands of magnesian lime- 

 stone near "Whitehaven, Stank, and Bispham. On the other hand, 

 the upper and lower sedimentary beds attain great dimensions ; the 

 former, as Sir Eoderick Murchison has shown, represented by the 

 St. Bees sandstone, over 600 feet in thickness ; and the latter by 

 the Penrith sandstone, estimated by Professor Harkness to attain a 

 thickness of 3000 feet. It is clear therefore that the sedimentary 

 region in the north of England area has been to the westward, and 

 the calcareous area to the eastward ; and that in this group there 

 has been a development from opposite directions of the two types . 

 of strata.t 



North-west of England. Norlh-cast of England. 

 FeH. Feet. 



Upper Permian (Sedimentary) .. COO .. .. 50 to 100 

 Middle „ (Calcareous) .. 10 to SO .. .. 600 



Lower „ (Sedimentary) .. 3000 .. .. 100 to 250 



I now pass on to the consideration of the physical conditions of 

 which these interchanges of vertical development between the cal- 

 careous and sedimentary strata are the palj)able representatives. 



Natural Grou^ping of Strata on a Threefcld System. — Ee- 

 gardiug the calcareous strata as the representative of the pelagic, 

 or deep-sea, conditions, and the sedimentary as the representative of 

 more littoral conditions, it is to be understood that each of the three 

 divisions of a natural group represents the 'predominance of these 

 conditions at each successive stage, and not their existence to the 

 exclusion of others. We must also take as representatives of a 

 special group that section of it ivhich is someiohut intermediate 

 hetween its extreme conditions of littoral and p)elagie ; for we can 

 easily understand that there must have been tracts in the ocean 

 where sediment was never deposited during (for instance) the whole 

 of the Carboniferous period, and where the only representative 

 of the series would be the Carboniferous limestone. And, on 

 the other hand, there must have been tracts bordering on the 

 old land surfaces of the period where the calcareous member was 

 scarcely represented (as in Scotland), and where sedimentary strata 

 were formed, almost to the exclusion of others. As a matter of 

 fact, however, it is this portion of the geological groups (between 



* As shown by Mr. E. W. Bianey, Mem. Lit. and riiil. Soc, Manchester, 

 vol. xii., &e. 



t I have not referred to the Lower Permian beds of the central counties, as the 

 relationship of these beds to those of the north is somewhat obscure. They were 

 proljably dejiosited in separate basins. 



